This invention relates to heliothermal energy conversion systems and, more particularly, to systems of the type wherein a heat transport fluid is forced to circulate between a solar energy collection area and a thermal energy utilization device.
Heliothermal energy conversion systems are known in the art and generally include a solar water heater or collector panel of the flat-plate collector type. The solar collector panels generally include an outer cover of glazing which may be one or more sheets of glass or other radiation-transmitting material. The collectors include inlet and outlet ducts or headers which are interconnected by conduits in the form of tubes or fins for conducting or directing the heat transfer fluid across the surface of a collector plate. The conduits are coupled to the collector plate in thermal contact therewith so that the incident solar energy heats the water flowing through the conduits. The underside of the collector plate is provided with a layer of insulation to minimize downward heat loss, and the entire solar panel is enclosed in a frame or casing which surrounds the above-mentioned components and permits solar energy to pass through the glazing material.
To date, such solar collector panels have been utilized in thermosyphone heaters comprising a storage vessel or tank mounted at a level higher than the level of the solar collector, and supply and return lines which connect the storage tank to the solar collector panel. The solar panels, which are disposed at an optimum angle to the incident radiation of the sun, function to heat the water in the conduits or tubes of the collector plate and the warm water flows upwardly through the collector panels and into the storage tank. This system utilizes the tendency of warmer water in a closed system to rise to a higher level than the colder water therein. However, where architectural or other considerations necessitate that the storage vessel or tank be mounted below or at a considerable distance from the solar collectors, a forced circulation system must generally be employed.
These prior art systems have several known disadvantages. For example, relatively large pumps must be provided in order to compensate for the fluid pressure drop appearing across the solar collector panels. Accordingly, the larger pumps increase the initial and operating costs of the system. Further, since the heated water is collected in a hollow homogeneous storage tank, the instantaneous hottest hot water is not always directly available for the heating and/or cooling functions. That is, during those periods when the sun's radiation is not strong enough to keep the collector panels hot, relatively cold water enters the storage tank; and, if the inlet is in close proximity to the energy utilization device, the hotter water in other portions of the tank is not available for the energy utilization device. Conversely, when the water being supplied by solar collector assembly is hotter than the average temperature of the water stored in the tank, the hottest water is also available to the utilization device as it is diluted by the other colder water.
Another known disadvantage of these prior art heliothermal systems is that a costly anti-freeze solution or additive must be provided to avoid any damage that might occur due to freezing in the system plumbing. Accordingly, the heat transfer between the solar collectors and the heat-transfer fluid is reduced due to the lower coefficient of heat transfer of the anti-freeze solution, and the use of common anti-freeze additives such as ethylene glycol can accelerate corrosion of system components. Further, the viscosity of these anti-freeze solutions is substantially higher than water alone and, accordingly, required a larger and more costly pump size. Additionally, anti-freeze solutions provide a significant disposal problem from an environmental consideration viewpoint; and, the danger of contamination of the source of domestic or potable water is undesirably present.
Another known disadvantage of these prior art systems is the difficulty of installation and maintenance of these solar collector panels due to their relatively large size and integral construction. That is, routine maintenance such as the replacement of a broken glazing panel necessitates, in many cases, a complex disassembly of the entire solar collector panel assembly.
These and other disadvantages are overcome by the present invention wherein there is provided a new solar energy conversion system of the type having a heat transport fluid circulating therein. In the system of the present invention, pressure drop across the solar collector panel is significantly reduced; and, means are provided for assuring that the hottest available water is presented to the thermal energy utilization device. Further, a vent line is provided between the solar collector panel(s) and the storage tank which functions to return or dump the fluid into the storage tank in the event that the ambient temperature at the solar energy collection area drops below a predetermined value and/or when the system pump is turned off. Thus, the danger of damage caused by water freezing in the solar collector panel and/or interconnecting lines is eliminated as is the need for anti-freeze solutions. Still further, the design of the solar collector panel assembly facilitates installation and maintenance as it is capable of being partially disassembled without necessitating breaking down the entire assembly.